7 Must-Know Truths About Due Dates

Bet on Unpredictability

As a journalist who's accustomed to meeting deadlines, I wasn't surprised when, during my first pregnancy, I went into labor on my due date. The second time around was another matter. My due date came and went. And went, and went, and went. It wasn't until the day before my doctor planned to induce me—at nearly 42 weeks pregnant—that I finally went into labor.

My experience brought home one of the essential truths of pregnancy: Your due date is anything but predictable. Still, your doctor needs to determine one that's as accurate as possible so that any necessary tests are done at the correct time. "And knowing how far along you are makes it easier for your obstetrician to see that your baby is growing properly," explains ob-gyn Joanne Stone, M.D., coauthor of Pregnancy for Dummies.

The Dating Game

To calculate your due date, your doctor will take the first day of your last menstrual period (LMP), and add 280 days (the equivalent of 40 weeks). For example, if your last period started on September 1, your due date would be June 7. This method assumes that your period arrives like clockwork every 28 days. If your cycles are longer, you're likely to deliver later than your due date; if they're shorter, expect to deliver earlier.

A first-trimester sonogram is more trustworthy. Your doctor measures the length of the fetus and size of the gestational sac. If these numbers don't match up with what your LMP would predict, she may adjust your due date.

But even if your doctor can pinpoint your due date with laserlike precision, don't expect your baby to show up right on schedule. Because, for all the windows into your womb, no one really understands what triggers labor. One possible (but, as my experience shows, imperfect) predictor: your past performance. "Patients tend to repeat what they did in a previous pregnancy," says Iffath Hoskins, M.D., executive director of the Women's Service at the Memorial Health University Medical Center, in Savannah, Georgia.

If you're a first-time mom, look at your mother's childbirth history. "If your mother delivered one week past her due date, you are more apt to deliver one week past your due date as well," explains Fredric D. Frigoletto, Jr., M.D., chief of obstetrics at Massachusetts General Hospital, in Boston.

Sooner Than Expected

Most women—about 80 percent—deliver sometime between 37 and 42 weeks. So where does that leave the other 20 percent? About 11 percent deliver prematurely. Experts aren't sure why some women go into labor early. Possible risk factors include carrying multiples or having an abnormally shaped uterus. There may also be a connection between preterm labor and an infection such as bacterial vaginosis. Even your build can play a role—women who weigh less than 110 pounds are at slightly higher risk.

Waiting It Out

Once you've passed your due date, you may wonder whether your baby will arrive later than 42 weeks, or post-term. While Caucasian women tend to have slightly longer pregnancies, as do women under 30, no one knows why some babies take their own sweet time. However, your baby's gender could be a clue: One study found that boy fetuses are more likely to go beyond their due date than girls.

Beginning at 40 or 41 weeks, expect to undergo weekly, then twice-weekly, nonstress tests. If your pregnancy progresses beyond this point, there's a risk that the quality of the placenta will deteriorate, the amniotic-fluid level will decline, or that your baby will pass a bowel movement, known as meconium. "Babies begin to practice breathing, and they can aspirate the meconium into their lungs—causing postdelivery problems," Dr. Frigoletto says. In addition, because your baby is still growing, you're at higher risk of needing a C-section. For these reasons, your doctor may want to induce labor once you hit 42 weeks.

Whether you give birth exactly on your due date—or in the weeks before or after—doesn't really matter in the long run. After all, you'll just be happy to meet your newborn whenever she arrives.

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